1. Field of the Invention
The present invention relates to a transmission type X-ray tube being reduced in size while retaining voltage resistivity.
2. Description of the Prior Art
In an X-ray tube, electrons emitted from a heating filament are accelerated by high voltage applied between a cathode and anode within a tube in high vacuum and collide with an anode target surface opposing the cathode, generating X-rays. X-ray tubes include such medical uses as CT scanning and such industrial uses as nondestructive inspections and thickness measurements. There are various constructions for this X-ray tube in the prior art, as disclosed in Japanese Laid-Open Patent Publications (Kokai) Nos. SHO-57-187848 and SHO-48-52390.
However, the above-described medical and industrial X-ray tubes are large, and with the current trend of reducing the size of such products as air cleaners there is a demand for smaller X-ray tubes, as well. It is possible to greatly reduce the size of X-ray tuber by depositing a focusing electrode on the inner surface of the bulb, which forms the X-ray tube. However, depositing a focusing electrode on the inner surface of the bulb decreases the space between the point at which the target and bulb meet and the point at which the focusing electrode and bulb meet. As a result, voltage resistivity cannot be maintained.